WX BRIEF

﻿﻿﻿﻿Wizards of storm forecasting

﻿

﻿By Karsten Shein
Comm-Inst, Climate Scientist

﻿﻿Forecasters monitor conditions and give media briefings at the NWS forecast office colocated with the Storm Prediction Center in Norman OK. Local forecast offices make use of SPC products and forecasts to deliver timely aviation weather information.

﻿Even though I have been a pilot and atmospheric scientist for many years, it still impresses me when I get a briefing with specific mention of thunderstorms or severe weather and, sure enough, at the forecast time and place, an anvil-topped cumulonimbus is poking up out of the haze to greet me.

It impresses me because I know just how complex the physics of thunderstorm formation can be. While we know the conditions necessary to create a thunderstorm, and can use that information to find general regions where they are likely to form, it is quite a bit more difficult to predict with accuracy how those conditions will develop over time, where exactly the convection is likely to kick off within the region, and what will become of a storm once it matures.

Fortunately, at least for pilots in the US, the National Oceanic and At­mos­pheric Administration (NOAA)'s National Weather Service (NWS) has a unit—the Storm Prediction Center (SPC)—which is home to a group of forecasters and weather modelers who are among the world's best at understanding severe weather.

The information provided by the SPC is used to issue Airmets and Sigmets, and it helps the briefers to give pilots a more accurate picture about severe weather potential along the route of flight. And, thanks to the Internet, pilots can be even more direct beneficiaries of a great deal of the information the SPC produces.

The SPC is located in Norman OK. Between the University of Oklahoma's School of Meteorology, the National Severe Storms Laboratory and the SPC, Norman is a mecca of severe weather research.

In fact, these 3 institutions are part of a broader consortium known as the National Weather Center, which also includes the Cooperative Institute for Mesoscale Meteorological Studies, the Atmospheric Radar Research Center, the Center for the Analysis and Prediction of Storms, and the Oklahoma University Supercomputing Center for Education and Research.

﻿Map of convective available potential energy (CAPE) over the US. CAPE values indicate instability in the middle troposphere. Values above 1500 generally support thunderstorms, while values over 2500 increase the potential of a thunderstorm being severe.

When you see a TV show or news story about storm chasing, more often than not it's a group of employees, professors and students from the National Weather Center who are doing the chasing. It is therefore no surprise that some of the most cutting-edge knowledge and computing resources are being brought to bear on providing the most accurate forecasts of severe weather available.

The main page of the SPC website—spc.noaa.gov—carries a tabbed image showing current activity across the US that is being tracked by the SPC. The overview merges the national weather radar mosaic and convective outlook areas. A convective outlook region is an area where there is a chance of convective activity. Convective outlooks are generally issued for the current day and 1–3 days ahead.

Across the top of the image on this main page are tabs which, when selected, display current severe weather watches and warnings, mesoscale discussions (MDs), storm reports, a mesoscale analysis, fire weather outlook and a county-based weather hazards image. On any of these images, a mouse click over an area of interest will take you to more information.

Convective outlooks

﻿Forecasters from NOAA's Storm Prediction Center ready a severe weather outlook. The SPC is responsible for forecasting and tracking conditions that may lead to severe weather events and outbreaks. Their forecasts can give a pilot enough time to divert around hazardous storms.

One of the most widely used SPC products is the convective outlook. SPC forecasters issue these for 1, 2, 3 and 4–8 days out. They are issued about every 6 hrs and the 1-day outlook covers the first 24 hrs after issuance. In a convective outlook, green shaded areas indicate the potential for thunderstorm activity, but little chance for severe weather.

If an area has a greater potential for the development of severe storms, that region will be shaded in yellow (slight chance), red (moderate chance), or purple (high chance). Pilots can use these outlooks as a quick reference and can modify their route of flight to avoid regions of elevated severe weather potential.

Following the graphical outlook is a textual forecast discussion that goes into more detail about the shaded risk regions and explains why they are shaded the way they are. Often, this discussion will give pilots a more refined explanation of why a region ought to be avoided.

For example, the discussion of an area of moderate risk for severe weather might indicate an expectation of isolated supercell development. Meanwhile, a nearby area of slight risk might include a call for frequent and widespread thunderstorms, even though few are expected to become severe.

Although both areas should be avoided if possible, the former may bring less chance of encountering a storm than the latter, even though there is a higher risk in the former of a storm becoming severe.

In addition, the text discussion may reveal that, even though an area has not been shaded for severe weather potential, severe conditions are not out of the question. For example, a recent graphical outlook showed only a thunderstorm risk over the Carolinas, but the discussion revealed that while most storms would be individual, a few could merge, leading to potentially "intense/persistent updrafts or storm clusters capable of [isolated] wet microbursts and wind damage."